The element besides uranium whose radioactivity Marie Curie observed in 1898 was thorium. While studying uranium minerals, Curie tested a wide range of substances to determine whether the radiation was unique to uranium or a broader atomic property.
Her measurements showed that thorium compounds emitted radiation resembling uranium’s emissions. She reported this result in April 1898, helping establish that radioactivity was associated with atoms of more than one element. Curie’s work was independent of a finding published about two months earlier by the German chemist Gerhard Carl Schmidt, who had also observed thorium’s radioactivity.
Thorium became important in the developing science of radioactive decay. Later researchers, including Ernest Rutherford, studied its emissions and discovered that thorium’s activity could vary because of short-lived radioactive products in its decay series. Curie’s early observation also strengthened her argument that radioactivity was an intrinsic property of matter, not a temporary effect caused by light or chemical reactions.